In many locations throughout the world there are formations near the earth's surface which have large quantities of hot water which can be used to produce mechanical energy. One way of utilizing this heat entails drawing the hot water from a natural spring or bringing it to the surface in a well and passing it through a heat exchanger, of the type capable of heating and vaporizing a fluid, in heat exchange relation with a working fluid of a character that it will be evaporated by such heat exchange. The working fluid is thus vaporized and sometimes superheated. The hot vapor is then expanded in an expansion engine to produce mechanical energy. It is then cooled and condensed so that it can be recycled. The water leaving the heat exchanger is returned to the same formation by means of a second well some distance from the producing well. This type of system absorbs most of the heat of the water at high temperatures as the preheat needed to bring the working fluid to its boiling point is usually only a small part of the total required heat.
One problem encountered in such systems is that of heat waste due to inefficient use of the hot water caused by the vaporization properties of the working fluids. Suppose the heat exchanger used is of the parallel countercurrent flow type. The temperature of the water at any point in such a heat exchanger must be higher than that of the working fluid at that point. Due to the latent heat of vaporization this temperature difference is usually very small at and slightly below the working fluid's initial boiling point and very large near the point at which the working fluid has absorbed its latent heat of vaporization and begins to superheat. Such small differences require excessive heat exchanger cost while the large ones cause power waste. For this reason, most such systems have used simple boilers as their heat exchangers. Boilers, however, are also inefficient because they are not capable of taking full advantage of the heat of the hot water and thus require large quantities of water.
One attempt to alleviate this situation might be to use the countercurrent type heat exchanger with a working fluid which requires a large amount of preheat and a small amount of latent heat of vaporization. This might alleviate the problem slightly, but has not been highly successful. There is still a "plateau" of constant temperature during vaporization of the working fluid which causes heat and power waste.
U.S. Pat. No. 3,516,248 to M. McEwen discloses the use of hydrocarbons as working fluids in such systems. These hydrocarbons present the problems described above. The McEwen patent states that it is possible to use mixtures of hydrocarbons in these systems. However, it does not recognize or deal with the above problems nor teach that it would be possible that suitable fluids could be chosen and so mixed as to alleviate these problems.